1. Field of the Invention
The present invention relates to deep UV lithography used in semiconductor manufacturing and more particularly to rework of multilayer photoresist systems.
2. Description of Related Art
The continuing drive for miniaturization of semiconductor devices has caused an increased rigor in the photolithography used to delineate the fine patterns of those devices. The demands for finer resolution have caused the shrinkage of imaging wavelengths from 365 nm (high pressure mercury lamp) to 248 nm (KrF excimer lasers), to 193 nm (ArF excimer lasers) and beyond. Because of this, the traditional lithographic materials, such as novolaks, diazonaphthoquinones, etc., are unsuitable platforms for ultra large scale integration (ULSI) manufacture and beyond. Thus, in order to utilize deep UV light wavelengths, new resist materials with low optical absorption and enhanced sensitivities were needed. As the patterns and wavelengths become finer, the materials properties of the photoresists used for pattern delineation have become more and more demanding. In particular, requirements of sensitivity, transparency, aesthetics of the image produced, and the selectivity of the resists to etch conditions for pattern transfer become more and more strenuous.
Chemically amplified resist materials have been developed through the use of acid-labile polymers in order to meet the above-mentioned criteria. They have shown great promise in increasing resolution. However, chemically amplified resist systems have many shortcomings. One problem is standing wave effects, which occur when monochromatic light is reflected off the surface of a reflective substrate during exposure. The formation of standing waves in the resist reduces resolution and causes line width variations. For example, standing waves in a positive resist have a tendency to result in a foot at the resist/substrate interface reducing the resolution of the resist. Frequently, resists are employed over a thin antireflective coating to minimize such problems.
In addition, chemically amplified resist profiles and resolution may change due to substrate poisoning. Particularly, this effect occurs when the substrate has a nitride layer. It is believed that residual N—H bonds in the nitride film deactivate the acid at the nitride/resist interface. For a positive resist, this results in an insoluble area, and either resist scumming, or a foot at the resist/substrate interface.
Furthermore, lithographic aspect ratios require the chemically amplified resist layer be thin, e.g., about 0.5 μm or lower, to print sub 0.18 μm features. This in turn requires the resist to have excellent plasma etch resistance such that resist image features can be transferred down into the underlying substrate.
One approach to solving the need for high resolution and high etch resistant resists involves the use of multilayer resist systems. In this approach, a thin, silicon-containing imaging layer is deposited over a thicker planarizing layer (underlayer). The underlayer absorbs most of the deep UV light attenuating standing wave effects. In addition, the underlayer prevents deactivation of the acid catalyst at the resist/substrate interface. Furthermore, the underlayer can contain some aromatic groups to provide etch resistance.
In the typical bilayer resist process, the undercoat layer is applied on the substrate. The chemically amplified resist is then applied on the undercoat layer, exposed to deep UV light and developed to form images in the chemically amplified resist topcoat. The bilayer resist system is then placed in an oxygen plasma etch environment to etch the undercoat in the areas where the chemically amplified resist has been removed by the development. In a subsequent step, the exposed substrate is plasma etched. The oxidized resist and the undercoat provide plasma etch resistance to the substrate etch.
Examples of bilayer imaging systems have been disclosed in commonly assigned U.S. Pat. Nos. 6,146,793; 6,165,682; and U.S. patent application Ser. No. 09/576,146, which are incorporated by reference herein.
During the manufacture of semiconductor devices it may become necessary to redo photoresist imaging steps because of mistakes or equipment problems. The photoresist imaging layer must be removed and the surface of the underlayer must be reprepared, recoated with photoresist and reimaged. The present invention overcomes the need to reprocess the underlayer, resulting in a more efficient overall manufacturing process.